Reducing cross-modulation in multichannel modulated optical systems

ABSTRACT

A modulated optical system with cross-modulation compensation reduces or corrects cross-modulation that might occur in a multichannel RF signal modulating a laser. The system detects the cross-modulation, for example, by detecting an envelope of the RF signal or by detecting RF power fluctuations, generates a cross-modulation detection signal, and imparts a compensating cross-modulation by adjusting a bias current of the laser in response to the cross-modulation detection signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application relates to U.S. patent application Ser. No.12/245,028 filed concurrently herewith, which is fully incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to modulated optical systems and moreparticularly, to a system and method for reducing cross-modulation inmultichannel modulated optical systems.

BACKGROUND INFORMATION

In a communications system where multiple channels are transmitted, suchas a CATV system, multiple analog signals corresponding to the multiplechannels may be combined into a wide-band multichannel RF signal, whichdrives a laser to produce a multichannel modulated optical signal. Themultiple analog signals may include multiple modulated analog carriersthat may be combined, for example, using frequency division multiplexingtechniques. One or more digital signals modulated using digitalmodulation, such as quadrature amplitude modulated (QAM), may also becombined with the modulated analog carrier signals, for example, usingsubcarrier multiplexing (SCM) techniques. In some systems, for example,as many as 110 channels may be transmitted over a frequency range ofabout 50 MHz to 750 MHz.

Cross-modulation occurs when the nonlinearities of a system result in acarrier in a multi-carrier system (i.e., a multichannel RF signal) beingmodulated by the various signals carried on other channels in the samesystem. In a CATV system, for example, a group of video carriers maymodulate other video carriers in a multichannel video system. Becauseeach video channel contains a constant, high-level signal component atthe horizontal line frequency (about 15.75 kHz in the NTSC system), thismay be the most noticeable component of cross-modulation. One source ofcross-modulation may be RF amplifiers in which gain compression producesless gain at higher RF signal power than at lower RF signal power.Another source of cross-modulation may be changes in bias current toreduce or prevent clipping in the laser, for example, as described ingreater detail in U.S. patent application Ser. No. 12/053,104 filed Mar.21, 2008, which is commonly owned and fully incorporated herein byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better understood byreading the following detailed description, taken together with thedrawings wherein:

FIG. 1 is a functional block diagram of a multichannel modulated opticalsystem with cross-modulation compensation, consistent with embodimentsof the present disclosure.

FIGS. 2A and 2B are functional block diagrams of multichannel modulatedoptical systems with cross-modulation compensation, consistent withother embodiments of the present disclosure.

FIG. 3A illustrates an example of a multichannel RF signal withcross-modulation.

FIG. 3B illustrates a cross-modulation detection signal generated inresponse to the multichannel RF signal shown in FIG. 3A, consistent withembodiments of the present disclosure.

FIG. 3C shows variation of the magnitude of a cross-modulation detectionsignal, consistent with embodiments of the present disclosure.

FIG. 3D shows variation of the phase of a cross-modulation detectionsignal, consistent with embodiments of the present disclosure.

FIG. 4 is a functional block diagram of a multichannel modulated opticalsystem with cross-modulation compensation and anti-clipping, consistentwith another embodiment of the present disclosure.

FIG. 5 is a schematic diagram of one embodiment of a phase controlelement that may be used in a multichannel modulated optical system withcross-modulation compensation.

DETAILED DESCRIPTION

Referring to FIG. 1, a modulated optical system 100 withcross-modulation compensation, consistent with embodiments describedherein, is capable of reducing cross-modulation by varying bias currentin response to cross-modulation detected on a multichannel RF signal.The modulated optical system 100 generally includes a laser 104 thatreceives a multichannel RF signal 106 from a multichannel RF source 102.The laser 104 may include a RF input that receives the multichannel RFsignal 106, a bias current input that receives a bias current 108, andan optical output that produces a modulated optical signal 109 inresponse to the RF signal 106 and the bias current 108. The system 100may also include other circuitry and/or components (not shown) betweenthe multichannel RF source 102 and the laser 104 such as, for example,one or more predistortion circuits and a laser driver circuit. In oneexample, the modulated optical system 100 may be an optical transmittersuch as a CATV transmitter.

By varying the bias current 108 in response to cross-modulation detectedon the multichannel RF signal 106, the system 100 may impartcompensating cross-modulation with a magnitude substantially equal tothe magnitude of the detected cross-modulation and with a phase that issubstantially opposite the phase of the detected cross-modulation,thereby compensating for the detected cross-modulation. As used herein,“compensate,” “compensation” or “compensating” for cross-modulationmeans reducing detected cross-modulation to a point that is tolerable ina particular system and does not necessarily require elimination ofcross-modulation.

The system 100 includes cross-modulation detection circuitry 110 thatdetects cross-modulation in the multichannel RF signal 106 from themultichannel RF source 102 and bias control circuitry 120 that controlsthe bias current 108 provided to the laser 104. The cross-modulationdetection circuitry 110 may be coupled to the RF signal path, forexample, using a splitter 107, such that the RF signal 106 is providedto the cross-modulation detection circuitry 110 and to the laser 104.The cross-modulation detection circuitry 110 produces a cross-modulationdetection signal 118 representing at least a portion of thecross-modulation on the multichannel RF signal 106.

The bias control circuitry 120 varies the bias current 108 in responseto the cross-modulation detection signal 118, which imparts compensatingcross-modulation on the RF signal 106 provided to the laser 104. Inother words, the cross-modulation detection signal 118 may be used tomodulate the bias current. Modulation of the bias current by thecross-modulation detection signal 118 causes fluctuations in the biascurrent, which cause the RF signal 106 to be modulated with compensatingcross-modulation. The bias control circuitry 120 may vary the biascurrent, for example, such that the bias current changes in an oppositedirection of the detected cross-modulation on the RF signal to impartthe compensating cross-modulation that cancels out the detectedcross-modulation.

The multichannel RF signal 106 may include multiple superimposedmodulated analog carriers at different frequencies. The multiplemodulated analog carriers may be modulated using modulation techniquesknown to those skilled in the art, such as amplitude modulation, and maybe combined using multiplexing techniques known to those skilled in theart, such as frequency division multiplexing. The multichannel RF signal106 may also include one or more digital signals modulated using digitalmodulation, such as quadrature amplitude modulation (QAM). The resultingmultichannel RF signal 106 occupies a bandwidth across the range offrequencies of the multiple modulated carriers. Those skilled in the artwill recognize that various modulation and multiplexing techniques maybe used to generate the multichannel RF signal 106.

In one embodiment, the multichannel RF source 102 may include headendequipment in a CATV system and the multichannel RF signal 106 may be adownstream CATV signal. Examples of downstream multichannel CATV signalsinclude 77 channels transmitted over a frequency range of about 50 MHzto 550 MHz and 110 channels transmitted over a frequency range of about50 MHz to 750 MHz. Each channel in a downstream multichannel CATV signalmay include a video carrier, a color subcarrier and an audio carrier.Other types of signals and frequency ranges may also be transmitted. Insome embodiments, the multichannel RF input signal occupies a bandwidthover a frequency range of about 50 MHz to 1000 MHz.

Cross-modulation can occur when nonlinearities result in a carrier orchannel in the multichannel RF signal 106 being modulated by the varioussignals carried on other channels in the same system. In a CATV system,for example, a group of video carriers may modulate other videocarriers. Because each video channel contains a constant, high-levelsignal component at the horizontal line frequency (about 15.75 kHz inthe NTSC system), this may be the most noticeable component ofcross-modulation.

In the exemplary embodiment, the multichannel RF signal 106, whichoccupies a bandwidth across the range of frequencies of the multiplemodulated carriers, directly modulates the laser 104. Each channel inthe multichannel RF signal 106 may be driven or modulated up to acertain optical modulation index (OMI) depending upon a desiredchannel-to-noise ratio (CNR). In one embodiment, the OMI of at leastsome of the channels may be at least about 4% and more specificallyabout 5%.

Referring to FIGS. 2A and 2B, modulated optical systems 200, 200′ withcross-modulation compensation are described in greater detail. Thesystem 200 generally includes a primary signal path 203 for carrying amultichannel RF signal 206 to a laser 204 and a secondary signal path205 for providing the cross-modulation compensation. The secondarysignal path 205 may be coupled to the primary signal path 203, forexample, using a splitter 207. In the embodiment shown in FIG. 2A, thesystem 200 includes envelope follower or detector circuitry 210 todetect cross-modulation on the RF signal 206. The envelope follower ordetector circuitry 210 receives the multichannel RF signal on thesecondary signal path 205 and generates a cross-modulation detectionsignal that follows an envelope of the multichannel RF signal 206. Thechanges in the envelope of the RF signal 206 may be indicative orrepresentative of cross-modulation on the RF signal 206.

The envelope follower or detector circuitry 210 may include circuitryknown to those skilled in the art for detecting an envelope of a highfrequency RF signal. In particular, the envelope follower or detectorcircuitry 210 may include envelope detection circuitry that has aresponse time fast enough to detect an envelope of a multichannel RFsignal. Envelope follower or detector circuitry 210 may be implementedusing known envelope detection circuitry such as, for example, aprecision rectifier circuit and a low pass filter. Specificimplementations of envelope detection circuitry capable of detecting theenvelope of such a signal are shown and described in U.S. patentapplication Ser. No. 12/053,104 and in U.S. patent application Ser. No.11/753,082, which are fully incorporated herein by reference.

The system 200 may also include one or more gain control elements 212,such as a variable gain element and/or a variable attenuator, to providegain and/or loss and thus controllably vary a magnitude of thecross-modulation detection signal generated by the envelope detectioncircuitry 210. The gain control element(s) 212 may provide gain or lossdepending upon how much compensating cross-modulation is needed tocompensate for the detected cross-modulation. In other words, the gaincontrol element(s) 212 may be used to provide gain or loss such that themagnitude of the compensating cross-modulation imparted to the RF signalcorresponds sufficiently to the magnitude of the cross-modulation beingcompensated.

The system 200 may further include one or more phase control elements214, such as a phase switch, to control a phase of the cross-modulationdetection signal generated by the envelope detection circuitry 210. Thephase control element(s) 214 may change the phase as needed (e.g., byswitching between positive and negative) to ensure that the phase of thecompensating cross-modulation imparted to the RF signal is sufficientlyout of phase with respect to the cross-modulation being compensated. Oneexample of a phase switch may be implemented using an inverted amplifiercircuit 500, as shown in FIG. 5, which is generally known to thoseskilled in the art. Although the phase control element 214 is shownfollowing the gain control element 212, the phase control element(s) 214and gain control element(s) 212 may be arranged differently on thesecondary signal path 205.

The system 200 further includes bias control circuitry 220 that adjustsor varies a bias current 208 in response to the cross-modulationdetection signal that follows the envelope of the RF signal 206 and thathas a magnitude and phase sufficient to compensate for the detectedcross-modulation. The laser 204 receives both the RF signal on theprimary signal path 203 and the varying bias current provided by thebias control circuitry 220 and generates a modulated optical signal inresponse thereto. When the varying bias current is combined with the RFsignal, compensating cross-modulation is imparted on the RF signal 206to compensate for the detected cross-modulation. The bias controlcircuitry 220 may also adjust the bias current in response to othersignals, such as an anti-clipping signal and/or a power monitor signal.The bias control circuitry 220 may include circuitry know to those ofordinary skill in the art for providing a bias current to a directlymodulated laser.

Because of the time required to detect the envelope and to generate thecross-modulation detection signal, a delay may exist between thecross-modulation detection signal on the secondary signal path 205 andthe RF signal on the primary signal path 203. As a result of the delay,the varying bias current may lag behind the RF signal. According to oneembodiment, the system 200 may also include a delay element (not shown)in the primary signal path 203 such that the RF signal is delayed toreduce or eliminate this lag. The delay element may not be needed,however, when the cross modulation is predominantly the 15.75 kHz signalcomponent because the RF delay is relatively short.

In the embodiment shown in FIG. 2B, the modulated optical system 200′includes an RF power detector 211 to detect the cross-modulation on theRF signal. The RF power detector 211 receives the multichannel RF signalon the secondary signal path 205, detects fluctuations in the RF power,and generates a cross-modulation detection signal that varies with thedetected power fluctuations. Similar to the envelope of the RF signal,fluctuations in the RF power may be indicative or representative ofcross-modulation on the RF signal. The RF power detector 211 may includecircuitry known to those skilled in the art for detecting powerfluctuations in a RF signal, such as a 50 MHz to 3 GHz monolithic RFpower detector capable of measuring RF signals over a 60 dB dynamicrange. One such RF power detector is available from Linear TechnologyCorporation under the name LT®5534.

In the system 200′, the magnitude and/or phase may then be adjustedusing gain control element(s) 212 and/or phase control element(s) 214 asdescribed above. The bias control circuitry 220 may then vary the biascurrent provided to the laser 204 in response to the cross modulationdetection signal that varies in response to RF power fluctuations.

FIGS. 3A-3D illustrate one example of the generation of across-modulation detection signal 318 from a multichannel RF signal 306.FIG. 3A illustrates a multichannel RF signal 306 with cross-modulation.The illustrated RF signal 306 includes periods 301 of lower power andperiods 303 of higher power. FIG. 3B illustrates the cross-modulationdetection signal 318 generated by cross-modulation detection circuitry,such as an envelope detector or RF power detector. The cross-modulationdetection signal 318 generally corresponds to the envelope or powerfluctuations of the RF signal 306 and is indicative of thecross-modulation on the RF signal 306.

FIG. 3C illustrates the cross-modulation detection signal 318 withchanges in magnitude that may be provided, for example, by the gaincontrol element(s) 212 shown in FIGS. 2A and 2B. Providing gain resultsin a cross-modulation detection signal 318 a with increased magnitude.Providing loss or attenuation results in a cross-modulation signal 318 bwith a decreased magnitude. As mentioned above, the magnitude of thecross-modulation detection signal 318 may be adjusted depending upon thecross-modulation to be compensated.

FIG. 3D illustrates the cross-modulation detection signal 318 withchanges in phase that may be provided, for example, by the phase controlelement(s) 214 shown in FIGS. 2A and 2B. Adjusting the phase results ina cross-modulation detection signal 318 c with a different phase thanthe cross-modulation detection signal 318. As mentioned above, the phaseof the cross-modulation detection signal 318 may be adjusted dependingupon the cross-modulation to be compensated. In an embodiment, the phasemay be changed such that the phase-adjusted cross-modulation detectionsignal 318 c is inverted relative to the cross-modulation detectionsignal 318. In other embodiments, the phase may be kept the same as thecross-modulation detection signal 318.

Referring to FIG. 4, a further embodiment of a modulated optical system400 with cross-modulation compensation may also provide anti-clipping.When multiple modulated carriers of a multichannel RF signal 406 alignin phase, the sum of the voltage of the aligned carriers may result in apeak voltage condition. When the optical modulation index (OMI) of eachchannel exceeds a certain level (e.g., exceeding about 3% OMI perchannel), the peak voltage condition may result in a higher occurrenceof negative voltage spikes or peaks that cause the laser input currentto fall below a threshold current of a laser 404, resulting in clipping.

The system 400 may include cross-modulation detection circuitry 410 thatprovides cross-modulation detection signal 418, for example, asdescribed above, and anti-clipping circuitry 430 that provides ananti-clipping signal 432. The cross-modulation detection circuitry 410and the anti-clipping circuitry 430 receive the RF signal 406 from amultichannel RF source 402 and generate the respective cross-modulationdetection signal 418 and anti-clipping signal 432 in response thereto.Bias control circuitry 420 may adjust a bias current 408 provided tolaser 404 in response to the cross-modulation detection signal 418and/or the anti-clipping signal 432. When the RF signal modulates thelaser 404, the varying bias current 408 may thus cause compensation ofcross-modulation, as described above, and/or a reduction of clipping. Asused herein, to reduce or correct clipping means to prevent one or morenegative spikes or peaks in the multichannel RF signal from causingclipping in the laser 404 and does not require a complete elimination ofclipping.

In one embodiment, the anti-clipping circuitry 430 includes envelopefollower circuitry that receives the multichannel RF signal 406, detectsan envelope of the RF signal 406, and generates the anti-clipping signal432 in response to the detected envelope. By adjusting the bias current408 in response to an envelope of the multichannel RF signal, the system400 reduces or corrects clipping that might occur in the laser 404 as aresult of negative spikes or peaks in the multichannel RF signal 406. Inone embodiment, the bias current 408 may be adjusted or varied inverselyproportional to a detected lower envelope of the multichannel RF signal.In particular, the bias current 408 may be increased as a lower envelopeof the multichannel RF signal 406 falls and indicates a peak negativevoltage condition. Thus, when the multichannel RF signal 406 falls belowthe threshold current of the laser as a result of a negative peak, thebias current 408 should be increased to a level that will prevent thatnegative peak and/or subsequent negative peak(s) from causing clipping.Embodiments of anti-clipping circuitry are described in greater detailin U.S. patent application Ser. No. 12/053,104 and in U.S. patentapplication Ser. No. 11/753,082, which are fully incorporated herein byreference.

Accordingly, systems and methods, consistent with embodiments of thepresent disclosure, may compensate for cross-modulation in amultichannel RF signal by varying the bias current to impart acompensating cross-modulation to the multichannel RF signal.

Consistent with one embodiment, a modulated optical system is providedwith cross-modulation compensation. The system includes a laserincluding a RF input configured to receive a multichannel RF signal, abias input configured to receive a bias current, and an optical outputconfigured to provide a modulated optical signal in response to themultichannel RF signal and the bias current. The multichannel RF signalincludes a superposition of multiple carriers, wherein at least one ofthe multiple carriers modulates at least one other of the multiplecarriers resulting in cross-modulation. The system also includescross-modulation detection circuitry configured to receive themultichannel RF signal and to generate a cross-modulation detectionsignal responsive to the multichannel RF signal. The system furtherincludes bias control circuitry coupled to the cross-modulationdetection circuitry and to the bias input of the laser. The bias controlcircuitry is configured to vary the bias current provided to the laserin response to the cross-modulation detection signal such that thevarying bias current imparts a compensating cross-modulation to the RFsignal compensating for at least a portion of the cross-modulation.

Consistent with another embodiment, a method is provided for reducingcross-modulation in a modulated optical system. The method includes:providing a multichannel RF signal to a laser diode, the multichannel RFsignal including a superposition of multiple carriers, and wherein atleast one of the multiple carriers modulates at least one other of themultiple carriers resulting in cross-modulation; receiving a portion ofthe RF signal on at least one secondary signal path; generating across-modulation detection signal responsive to the multichannel RFsignal on the secondary signal path; varying a bias current provided tothe laser diode in response to the cross-modulation detection signalsuch that the varying bias current imparts a compensatingcross-modulation to the RF signal compensating for at least a portion ofthe cross-modulation; and providing a modulated optical signal from thelaser.

While the principles of the invention have been described herein, it isto be understood by those skilled in the art that this description ismade only by way of example and not as a limitation as to the scope ofthe invention. Other embodiments are contemplated within the scope ofthe present invention in addition to the exemplary embodiments shown anddescribed herein. Modifications and substitutions by one of ordinaryskill in the art are considered to be within the scope of the presentinvention, which is not to be limited except by the following claims.

What is claimed is:
 1. A modulated optical system with cross-modulation compensation, the system comprising: a laser including a RF input configured to receive a multichannel RF signal, a bias input configured to receive a bias current, and an optical output configured to provide a modulated optical signal in response to the multichannel RF signal and the bias current, the multichannel RF signal including a superposition of multiple carriers, and wherein at least one of the multiple carriers modulates at least one other of the multiple carriers resulting in cross-modulation; cross-modulation detection circuitry configured to receive the multichannel RF signal and to generate a cross-modulation detection signal responsive to the multichannel RF signal; and bias control circuitry coupled to the cross-modulation detection circuitry and to the bias input of the laser, the bias control circuitry being configured to vary the bias current provided to the laser in response to the cross-modulation detection signal such that the varying bias current imparts a compensating cross-modulation to the RF signal compensating for at least a portion of the cross-modulation.
 2. The system of claim 1 wherein the cross-modulation detection circuitry includes envelope follower circuitry configured to receive the multichannel RF signal, to detect an envelope of the multichannel RF signal and to generate a cross-modulation detection signal that follows at least a portion of the envelope of the multichannel RF signal.
 3. The system of claim 1 wherein the cross-modulation detection circuitry includes RF power detector circuitry configured to receive the multichannel RF signal, to detect power of the multichannel RF signal and to generate a cross-modulation detection signal responsive to detected power fluctuations of the multichannel RF signal.
 4. The system of claim 1 further comprising at least one gain control element configured to adjust gain and/or loss of the cross-modulation detection signal such that the compensating cross modulation has a magnitude that compensates for the at least a portion of the cross-modulation.
 5. The system of claim 1 further comprising at least one phase control element configured to adjust the phase of the cross-modulation detection signal such that the compensating cross-modulation has a phase that compensates for the at least a portion of the cross-modulation.
 6. The system of claim 1 further comprising anti-clipping circuitry configured to receive the multichannel RF signal and to generate an anti-clipping signal in response to the multichannel RF signal, and wherein the bias control circuitry is configured to vary the bias current in response to the anti-clipping signal such that at least one negative voltage spike in the RF signal is prevented from causing clipping in the laser.
 7. The system of claim 6 wherein the anti-clipping circuitry comprises envelope follower circuitry configured to receive the multichannel RF signal, to detect an envelope of the multichannel RF signal and to generate an anti-clipping signal that follows at least a portion of the envelope of the multichannel RF signal.
 8. The system of claim 6 wherein the anti-clipping circuitry comprises peak detection circuitry.
 9. The system of claim 1 wherein the multichannel RF signal includes a CATV signal including a plurality of video channels.
 10. A method of reducing cross-modulation in a modulated optical system, the method comprising: providing a multichannel RF signal to a laser diode, the multichannel RF signal including a superposition of multiple carriers, and wherein at least one of the multiple carriers modulates at least one other of the multiple carriers resulting in cross-modulation; receiving a portion of the RF signal on at least one secondary signal path; generating a cross-modulation detection signal responsive to the multichannel RF signal on the secondary signal path; varying a bias current provided to the laser diode in response to the cross-modulation detection signal such that the varying bias current imparts a compensating cross-modulation to the RF signal compensating for at least a portion of the cross-modulation; and providing a modulated optical signal from the laser.
 11. The method of claim 10 wherein generating a cross-modulation detection signal comprises detecting an envelope of the multichannel RF signal such that the cross-modulation detection signal follows at least a portion of the envelope of the multichannel RF signal.
 12. The method of claim 10 wherein generating a cross-modulation detection signal comprises detecting power of the multichannel RF signal such that the cross-modulation detection signal is generated responsive to the detected power of the multichannel RF signal.
 13. The method of claim 10 further comprising adjusting a magnitude and/or phase of the cross-modulation detection signal such that the compensating cross-modulation has a magnitude and phase that compensates for the portion of the cross-modulation on the RF signal.
 14. The method of claim 10 further comprising: generating an anti-clipping signal responsive to the multichannel RF signal on the secondary signal path; and varying the bias current provided to the laser diode in response to the anti-clipping signal such that at least one negative voltage spike in the RF signal is prevented from causing clipping in the laser.
 15. The method of claim 10 wherein the multichannel RF signal is a CATV signal including a plurality of video channels. 